The invention relates generally to stator permanent magnet electrical machines and, more particularly, to a system and method for setting the operating point of permanent magnets of stator permanent magnet machines so as to avoid demagnetization of the magnets during a high temperature Vacuum Pressure Impregnation (VPI) process.
The usage of electrical machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. In manufacturing such electrical machines, a process known as Vacuum Pressure Impregnation (VPI) is an essential process for the windings of the electrical machine, as the VPI process improves the dielectric strength of the insulation on the windings as well as the winding thermal conductivity. The VPI process fills any voids or porosity in the winding with a high thermal conductivity varnish. In this way, the VPI prevents the apparition of air pockets and provides a thermally conductive contact between the copper wires, slot liners and the laminations. Hence, the VPI process significantly improves the thermal performance of the electrical machine and is therefore key in enhancing its life and reliability.
It is known that, in performing a VPI process, a high temperature cure of 150 C (or higher) is typically employed. In many types of conventional electrical machines, no special precautions need to be taken during the VPI process, as there is no special adverse effect of the high temperature cure. However, for some types of electrical machines—specifically for electrical machines that are designed to have permanent magnets positioned on the stator (i.e., “stator permanent magnet machines”—including permanent magnet flux switching machines, permanent magnet flux reversal machines, and doubly-salient permanent magnet machines, for example), the high temperature cure employed by the VPI process may expose the permanent magnets on the stator to a demagnetization risk if no special precaution is taken to ensure the load line of the permanent magnets are above the demagnetization knee for the cure temperature. This risk of demagnetization is further increased when the permanent magnets in the machine are in the form of low cost, low thermal stability magnet grades—such as Dysprosium-free or Reduced Dysprosium permanent magnets. That is, for the same temperature, the demagnetization knee occurs at higher internal flux density levels in Dysprosium-free or reduced Dysprosium Neodymium magnets as compared to their conventional Neodymium counterparts, which makes the demagnetization risk more serious in these low cost but low thermal stability reduced rare earth magnets.
Therefore, it would be desirable to provide a system and method for setting the operating point of permanent magnets of stator permanent magnet machines so as to avoid demagnetization of the magnets during a high temperature VPI process.